An accurate local model for triple substitutions in fourth order Møller–Plesset theory and in perturbative corrections to singles and doubles coupled cluster methods

Two noniterative local models for evaluating the contribution of triple substitutions to the electron correlation energy (as needed in MP4 and CCSD(T)), are developed. The occupied space is spanned by a minimal basis, and the virtual space by an extended basis of atom-centered functions. The triple substitutions are truncated by an atomic criterion such that either zero or one electrons can be transferred between atoms. The covalent model asymptotically recovers 70% of the triples correlation energy for poly-ynes with a 6-31G* basis, while the singly-ionic model recovers 99%.